This paper presents a novel microstrip quad-channel diplexer. The introduced diplexer operates at close resonance frequencies of 1.68 GHz, 1.9 GHz, 3.9 GHz and 4.43 GHz for multi-service wireless applications. The frequency responses of S21 and S31 are sharp at the edges of all passbands. To improve the stopband properties, several transmission zeros are created. Also, the low insertion losses of 0.68 dB, 0.57 dB, 0.71 dB, and 0.5 dB are obtained at the resonance frequencies. The proposed diplexer has a new structure with a compact size of 0.051 × λg 2. To verify the design method and simulation results, the proposed structure is fabricated and measured. There is a good agreement between the simulation and measurement results.
